Number stations can be a lot of different things.. most commonly believed to be part of various espionage efforts. The Soviets in the era of the Cold War particularly used Morse Code to transmit their number sequences... Also various services like air defense systems and various long range communications use them.. the codes transmitted are extremely difficult to decipher, most often it is a one time encryption, signified by an early sequence of numbers.. which hold the key to the rest of the sequence... and only the intended recipient has the "unlock" code.

Of course, it could not be a code at all. Sometimes, the sleeper agent is given one number or sequence, memorizes it, and performs his mission the moment he hears it, and the other numbers could just be gibberish.However, there is a cipher called the book cipher (I think) in which the words in a book or specially prepared text is numbered, and the numbers represent the first letter of the word they are assigned. For example, if 3 was assigned to "water", whenever the station broadcasted "3", it would mean "w".